Treatment of aluminum foil and
product produced thereby



United States Patent 26,892 TREATMENT OF ALUMINUM FOIL AND PRODUCT PRODUCED THEREBY William K. Hooper, Brookfieltl, Conn., assignor to Republic Foil, 1uc., Danbury, Conn., a corporation of Delaware N0 Drawing. Original No. 3,351,442, dated Nov. 7, 1967,

Ser. No. 592,962, Oct. 21, 1966, which is a continuation-in-part of applications Ser. No. 423,308, Jan. 4, 1965, and Ser. No. 463,055, June 10, 1965. Application for reissue Oct. 11, 1968, Ser. No. 769,456

Int. Cl. Hlllg 9/04 US. Cl. 29-183.5 11 Claims Matter enclosed in heavy brackets II] appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE [Aluminum foil electrodes having increased capacity for use in electrolytic capacitors are provided by cold working aluminum foil to reduce its original thickness by between 50% to 99% folowed by etching and then by annealing and anodic formation] Foil of aluminum or aluminum-base alloys which foil has been cold worked, is processed, to increase its capacitance for use in electrodes in electrolytic capacitors, by being etched and then subjected to anodi'c formation without being annealed prior to the etching.

This is a continuation-in-part of copending applications Ser. No. 423,308, filed Jan. 4, 1965, and Ser. No. 463,055, filed J one 10, 1965, both now abandoned.

The present invention relates to aluminum foil of increased capacity for use in electrolytic capacitors and to a method of making the same.

In the preparation of aluminum foil for use in electrolytic capacitors the electrical industry is constantly attempting to increase the efficiency of capacitors so as to obtain higher capacitance Without a simultaneous increase in the size of the capacitor.

Up to the present, it is customary to roll the aluminum foil into very thin sheets whereby the foil becomes cold worked and its strength is essentially increased. In operating methods used up to the present, the foils were then subjected to an annealing step in order to remove the effect of the cold worked surface. This step was considered essential before the etching of the aluminum took place, which is customary for the purpose of providing an in creased surface with attendant increase in capacitance of such aluminum foils. The etching may be either performed by chemical or electrochemical methods. Both these methods cause a considerable increase in the surface area of the aluminum which later undergoes oxidation by anodic formation.

It has now been discovered that a superior product can be obtained when the aluminum foil, after rolling, is directly subjected to an etching process without being first annealed. Thus, for the first time, it is possible not only to simplify the process of making etched aluminum foils for use in capacitors, but to obtain even better results by omitting the annealing step after rolling, thereby accomplishing large savings in time, labor and energy, and rendering the process especially economical.

While the aluminum foil etched directly after cold working exhibits particularly high capacitance when used in electrolytic capacitors, the foil loses ductility and cannot be shaped. Annealing is, therefore, necessary prior to use in capacitors of small size comprising cylindrically shaped coil electrodes.

It is the object of the present invention to provide aluminum foil to be used as an electrode in electrolytic capacitors, which is capable of acquiring an increased capacitance, superior to the best electrodes now available, by up to about 2050%.

Other objects and advantages of the invention will become apparent from the following detailed description.

As a rule, aluminum foil to be used as an electrode in electrolytic capacitors is used in thicknesses ranging from .0006 to .006 inch, the preferred range for cathode foil being from 0.0006 to 0.002 inch and the preferred range for anode foil being from 0.0025 to 0.004 inch. In solid state technology the preferred thickness is of an inch.

In carrying out the invention on aluminum foil of suitable thickness to yield gauge needed after reduction is treated by reducing its thickness by rolling into very thin sheets of 50 to 99% reduction of the original thickness thus obtaining a preferred orientation rolling texture. This texture consists essentially of planes coincident with the rolling plane and 1l2 direction coincident with the long dimension of the sheet. The aluminum foil is subsequently subjected to a degreasing treatment to remove all traces of rolling oil. Thereafter, the foil is subjected to electrochemical etching preferably in a sodium chloride electrolyte with the electrolyte concentration varying in the range of 5% to 30% sodium chloride, current density in the range of 0.7 to 10 amps per square inch, and coulomb input from 470 to 940. The etched foil is rinsed with de-ionized water, annealed by heating to temperatures between 450" C. and 550 C. and is then passed on to a bath in which anodic formation takes place at a voltage depending on the specific application.

In a particularly advantageous embodiment, the etched foil is rinsed with de-ionized water and is fed into high temperature ovens to effect air oxidation; in the ovens, a temperature as stated above ranging from 450 C. to 550 C. is maintained. The foil is advanced through the oven chamber at a speed which will allow it to remain in the zone of elevated temperature for a mean time of 2-10 minutes before being passed onto anodic formation. Annealing takes place in this case automatically during the oxidation step.

The following block diagram illustrates the sequence of operations:

Formation lgiiiiig Degrefising Etching Ringi g Alr gygltllggl igl g Anodic Formation In the following, a number of examples are given for illustrating a method according to the invention. The examples are partly arranged in tabulated form.

The samples of aluminum foil were treated as described above. One series of tests was carried out with formation of the foils in an electrolyte at 6 volts, another at 25 volts,

yet another series at 75 volts, and a fourth with a forma- As stated before, a current density in the range of 0.7 to 10 amps per square inch, may be applied, best results being accomplished at densities between 9 and 10.

The ultimate tensile strength of the foil increases from about 6000-7000 psi. for the annealed to about 15,000- 22,000 psi. for the cold Worked aluminum foil proposed by the process of this invention.

While the invention has been described with reference to an aluminum foil, it should be understood that aluminum-base alloys containing in excess of 95% by weight of Al and known to be useful as electrode material in electrolytic capacitors may likewise be subjected to the treatment according to the invention with the beneficial results described for aluminum proper. Such alloys are known in the capacitor trade, and they may contain e.g.

TABLE-ILLUSTRATIN G CAPACITANCE Reduction of Capacitance in Inf. Foil Material Thickness, Volts Coulomb Current Percent Density A B o Hardened Al 99.99810 Pure. 0 705 0. 30 155 220 280 Do 0 410 0.30 145 155 205 Do 75 470 0. 30 15. s0 42 Do 75 705 0. 30 10. 5 25 32 705 0. 30 1s. 0 31 705 0. 30 17.0 20 33 470 0. 30 17. s 23. 5 30 0.30 14. 5 21. 7 20 0 0. 30 14. 0 24. 0 20. 0 Do 00 105 470 0. as 4. 7 5. 00 7. 5 0.30 4.5 5. 00 7.0 705 0.30 4. 0 s. 1 11. 5 705 0. a0 5. 2 8.3 10. 7 0. 30 5.1 8.0 11.2 110 so 470 9. 14. 0 22 30 Do 00 25 470 0. 30 1s. 5 17 22 Both columns B and C ShOW considerable increase of Zn, Mg, Si, Mn, Cu, Fe and others. The following table capacitance in capacitors using the aluminum fo1l pregives some typical compositions for such alloys:

Alloy Copper Magnesium Silicon Iron Manganese Titanium Zine nl pared according to the invention, of Course, Showing 65 These alloys were likewise tested for increase in capacibest results. The best gains are obtained 1n the low and tance. In the following the values found for Alloy 1188 intermediate voltage ranges, that is, 25 to 165 volts, with are given by way of example: aluminum foils reduced in thickness to about 96%. However, reduction to between and will also make Reduction Capaciossible gain in capa itan to abo t 2()% Alloy o1 Thick- Volts Coulomb Current taneo in The gains obtained are a consequence of the increase of 35221 Density anodic sites for initiation of corrosive attack in the material. These anodic sites are brought about by increase 1188 96 25 470 6 7&0 of dislocation densities introduced in the material in the 1185 96 470 6 -5 cold worked process. 7 5

These figures were obtained when operations were carried out according to block diagram 2. As compared therewith, conventional working showed values by about 20 to 40% lower.

The foregoing disclosure relates only to preferred embodiments of the invention which is intended to include all changes and modifications of the examples described within the scope of the invention as set forth in the appended claims.

What is claimed is:

l. A method for improving the capacitance of foils for use as an electrode in an electrolytic capacitor comprising cold working a metal foil selected from the group consisting of aluminum and aluminum-base alloys containing in excess of 95% by weight of aluminum to reduce its original thickness by between about 50% to about 99% to a thickness between 0.0006 to 0.006 inch and subjecting the cold-worked foil to etching with subsequent annealing and anodic formation.

2. The method according to claim 1, wherein original thickness is reduced by between about 85% to about 96%.

3. A foil suitable for use as an electrode in an electrolytic capacitor comprising a metal foil selected from the group consisting of aluminum and aluminum-based alloys containing in excess of 95% by weight of aluminum, said metal foil having been subjected to cold working, etching, annealing and anodic formation in sequence and having a thickness reduced by 50 to 99% from an original thickness to 0.0006 to 0.006 inch, a tensile strength of 15,000 to 22,000 p.s.i.. and an increased capacitance of between 20 and 50% as compared to conventionally processed capacitor foils.

4. A foil according to claim 3 wherein said reduction is by 85 to 96% and said increased capacitance is between 40 and 50%.

5. The method according to claim 2, wherein the foil is subjected to a degreasing treatment after the rolling step.

6. The method according to claim 1, wherein after working in cold state and etching the foil is subjected to high temperature air oxidation and anodic formation.

7. The method according to claim 1, wherein said foil after cold working to a thickness reduced by about 50% to about 99% of the original thickness, is subsequently degreased to remove all traces of lubricating oil, then subjected to electrochemical etching in a sodium chloride electrolyte with a concentration varying from about 5% to [25%] 30% sodium chloride, current density in the range of 0.7 to 10 amps per square inch and coulomb input from 470 to 940, rinsed, subjected to high temperature air oxidation accompanied by annealing, and then subjected to anodic formation.

8. A method for improving the capacitance of a foil for use as an electrode in an electrolytic capacitor, said foil consisting essentially of aluminum or an aluminumbase alloy containing in excess of 95% by weight of aluminum and having been cold worked to reduce its original thickness by from about to about 99% of its original thickness to a thickness of from 0.0006 to 0.006 inch, comprising etching the cold worked foil and then subjecting the etchcd foil to annealing and anodic formation.

9. A method according to claim 8, in which the original thickness had been reduced by from about to about 96 10. A method according to claim 8, in which prior to said anodic formation the etched foil is subjected to high temperature air oxidation.

11. A method according to claim 8, in. which said etching is electrochemical and is conducted in a sodium chloride electrolyte with a concentration varying from about 5% to 30% sodium chloride, current density in the range of 0.7 to 10 amps per square inch, and coulomb input from 470 to 940.

References Cited 3/1960 Burger et al 204l41 7/1965 Vincent 204-141 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner 

